In general electrophotography, an electrostatic latent image is formed through various means on a photoreceptor which comprises a photoconductive substance, such as selenium, and a toner is deposited on the latent image by magnetic brush development or a similar technique to thereby make the latent image visible.
In the process of development, a carrier is used to impose an appropriate positive or negative charge on the toner particles. Various types of carriers have been developed and put to practical use.
Among various performance characteristics required for the carrier, particularly important are appropriate electrostatic charging properties, impact resistance, abra-sion resistance, developing properties and working life. Taking these characteristic requirements into consideration, conventional carriers still leave room for improvement, and none of them is quite satisfactory. For example, although conductive carriers, such as iron oxide particles, are superior in solid developability, they are inferior in reproducibility of fine lines and, in addition, require the presence of a special charge controlling agent for obtaining a prolonged working life of developer. On the other hand, coated insulating carriers are excellent in working life and fine line reproducibility but poor in solid reproducibility.
In an attempt to overcome these disadvantages, small-sized carriers comprising a binder resin having magnetic fine powders dispersed therein, i.e., so-called microtoning carriers, have been proposed and put into practical use. However, a carrier of this type undergoes adhesion to a photoreceptor because of its small particle size. Adhesion to a photoreceptor might be prevented by increasing the particle size of the carrier but, in turn, an increased size brings about reduction of charging properties, which leads to fog, or contamination of the interior of a developing machine. Moreover, magnetic particles released out of the surface of the carrier cause variation in the quantity of electric charge according to changes in humidity, and also drastic prolongation of life cannot be achieved due to difficulty in carrier surface treatment.